The foot pocket of the conventional thermoplastic swim fin is composed of thermoplastic rubber while the beams and the blade are composed of relatively hard thermoplastic elastomer. One of the major drawbacks of plastic fins is the tendency of the thermoplastic rubber material to sag on exposure to warm temperatures. Most manufacturers recommend that plastic shoe trees, packed with the fins, be inserted anytime the swim fins are not on the divers feet, but even shoe trees do not prevent warping of the foot packet.